1. Field of the Invention
The present invention relates generally to image scanning, printing and similar imaging systems in which image processing operations can be performed and, more specifically, to identifying and removing background noise and colors so that the remainder of the image appears cleaner.
2. Description of the Related Art
So-called “multifunction” or “all-in-one” machines that perform some combination of scanning, printing, copying, facsimile and other tasks are increasingly popular for home and business use. A user can place a document in the machine and, for example, press a “Copy” button to cause the machine to scan the document and print a copy.
Scanned images may be categorized into two subgroups, “content” and “background”. The content of a document refers to those elements of the image like text or pictures that are intended to be the information of interest to a reader. While the background is made up of everything else that is not content. For example, if black text is printed on yellow paper, the black text is the content while the yellow is the background. Additionally, scanned images inherently contain noise that causes undesirable blemishes in both the content and the background. A document copy consisting of text or pictures (i.e., content only) printed on a pure white background (i.e., a background having no noise) is desirable because it is more readable.
Methods have been developed to remove background colors and noise from a digitized image. Most such methods are based upon histograms. A processor reads the data pixel by pixel and constructs a histogram that reveals the most frequently occurring pixel value. The most frequently occurring pixel value is likely to be that of the background, because the background of a document typically occupies the greatest total area, with the text or pictures occupying a lesser total area. The most frequently occurring pixel value is also likely to be higher than the values of information pixels, because text and pictures are generally printed on white or light-colored backgrounds. Therefore, most background-removal methods simply identify the most frequently occurring pixel, and set to white all pixels having approximately that value or greater. Noise removal methods target the darker pixels of an image, especially in the case of a text document. By making a dark pixel darker (clipping) some noise can be removed.
Background-removal methods can be implemented in hardware or software, but in all-in-one machines, where speed is important to a user, implementing background-removal and other image processing in hardware, such as an application-specific integrated circuit (ASIC), is desirable. An important consideration in ASIC design is minimizing the amount of logic. Therefore, it would be desirable to minimize background-removal logic.
Most all-in-one machines scan documents into RGB color space. Therefore, it would be desirable for a background-removal method to function in this color space or at least be able to accept RGB data as input.
A more general statement of the background removal problem is the problem of determining how colorful a pixel is. If the colorfulness of a pixel can be quantified, the machine can process pixels according to their color. For example, a color background can be removed.
It would be desirable for all-in-one machines and other devices to be able to determine the colorfulness of a pixel and to perform background-removal and other functions based upon the colorfulness determination, a histogram, or both. It would further be desirable for such operations to be implemented efficiently in ASIC hardware. The present invention addresses these problems and others in the manner described below.